FR: Add MANDATORY state in MFA config

### [REQUIRED] Step 2: Describe your environment

* Operating System version: _____
* Firebase SDK version: _____
* Library version: __4.13.0___
* Firebase Product: __auth___ (auth, database, storage, etc)

### [REQUIRED] Step 3: Describe the problem

For the Multi-factor config state only `Enable` and `Disable` is supported, can we also add `MANDATORY` here shown from this [link](https://cloud.google.com/identity-platform/docs/reference/rest/v2/projects.tenants#mfastate).

Start with the Firebase Admin Go authentication code that defines the multi-factor configuration state, using the linked Identity Platform REST reference to confirm the MANDATORY value. Check how the existing Enable and Disable states are represented and tested; done means MANDATORY is accepted wherever those states are supported.

Written by the indexing model from the issue text.